Cbfa1 is a positive regulatory factor in chondrocyte maturation

Effects of Germanium embedded fabric on the chondrogenic differentiation of adipose derived stem cells

Osteoarthritis (OA) is a clinical state which is identified by the degeneration of articular cartilage. OA is a common condition (>500 millions of people affected worldwide), whose frequency is anticipated to continue to rise (> 110 % increase worldwide since 2019). The treatment for early-stage OA is based on a combination of therapeutic approaches, which can include regenerative medicine based on Adipose Derived Stem Cells (ADSCs). Germanium embedded Incrediwear® functional Cred40 fabric has been shown to have positive effects on OA clinically and is envisaged to give encouraging effects also on tissue regeneration. Still, the biological mechanisms underlying this therapeutic modality have not yet been fully defined. We tested the hypothesis that Germanium-embedded Incrediwear® functional Cred40 fabric could enhance chondrogenic differentiation. To this purpose, we applied Incrediwear® to human adipose-derived stem cells (hADSCs) induced to chondrogenic differentiation in vitro. Chondrogenic markers (ACAN, SOX9, RUNX2, COL2A1, COL10A1) were quantified following 21 days of treatment. We also assessed extracellular matrix (ECM) deposition (specifically Collagen and glycosaminoglycans (GAGs)) using Alcian Blue and Sirius Red staining. Here, we provide pilot data to demonstrate that Germanium-embedded Incrediwear® functional Cred40 fabric can enhance hADSCs chondrogenic differentiation and maturity and potentially induce events of cartilage regeneration.

Effects of Different Surface Functionalizations of Silica Nanoparticles on Mesenchymal Stem Cells

Physicochemical properties of nanoparticles, such as particle size, surface charge, and particle shape, have a significant impact on cell activities. However, the effects of surface functionalization of nanoparticles with small chemical groups on stem cell behavior and function remain understudied. Herein, we incorporated different chemical functional groups (amino, DETA, hydroxyl, phosphate, and sulfonate with charges of +9.5, + 21.7, -14.1, -25.6, and -37.7, respectively) to the surface of inorganic silica nanoparticles. To trace their effects on mesenchymal stem cells (MSCs) of rat bone marrow, these functionalized silica nanoparticles were used to encapsulate Rhodamine B fluorophore dye. We found that surface functionalization with positively charged and short-chain chemical groups facilitates cell internalization and retention of nanoparticles in MSCs. The endocytic pathway differed among functionalized nanoparticles when tested with ion-channel inhibitors. Negatively charged nanoparticles mainly use lysosomal exocytosis to exit cells, while positively charged nanoparticles can undergo endosomal escape to avoid scavenging. The cytotoxic profiles of these functionalized silica nanoparticles are still within acceptable limits and tolerable. They exerted subtle effects on the actin cytoskeleton and migration ability. Last, phosphate-functionalized nanoparticles upregulate osteogenesis-related genes and induce osteoblast-like morphology, implying that it can direct MSCs lineage specification for bone tissue engineering. Our study provides insights into the rational design of biomaterials for effective drug delivery and regenerative medicine.

Runx2 and Polycystins in Bone Mechanotransduction: Challenges for Therapeutic Opportunities

Bone mechanotransduction is a critical process during skeletal development in embryogenesis and organogenesis. At the same time, the type and level of mechanical loading regulates bone remodeling throughout the adult life. The aberrant mechanosensing of bone cells has been implicated in the development and progression of bone loss disorders, but also in the bone-specific aspect of other clinical entities, such as the tumorigenesis of solid organs. Novel treatment options have come into sight that exploit the mechanosensitivity of osteoblasts, osteocytes, and chondrocytes to achieve efficient bone regeneration. In this regard, runt-related transcription factor 2 (Runx2) has emerged as a chief skeletal-specific molecule of differentiation, which is prominent to induction by mechanical stimuli. Polycystins represent a family of mechanosensitive proteins that interact with Runx2 in mechano-induced signaling cascades and foster the regulation of alternative effectors of mechanotransuction. In the present narrative review, we employed a PubMed search to extract the literature concerning Runx2, polycystins, and their association from 2000 to March 2024. The keywords stated below were used for the article search. We discuss recent advances regarding the implication of Runx2 and polycystins in bone remodeling and regeneration and elaborate on the targeting strategies that may potentially be applied for the treatment of patients with bone loss diseases.

Extremely low frequency-electromagnetic fields promote chondrogenic differentiation of adipose-derived mesenchymal stem cells through a conventional genetic program

Progressive cartilage deterioration leads to chronic inflammation and loss of joint function, causing osteoarthritis (OA) and joint disease. Although symptoms vary among individuals, the disease can cause severe pain and permanent disability, and effective therapies are urgently needed. Human Adipose-Derived Stem Cells (ADSCs) may differentiate into chondrocytes and are promising for treating OA. Moreover, recent studies indicate that electromagnetic fields (EMFs) could positively affect the chondrogenic differentiation potential of ADSCs. In this work, we investigated the impact of EMFs with frequencies of 35 Hertz and 58 Hertz, referred to as extremely low frequency-EMFs (ELF-EMFs), on the chondrogenesis of ADSCs, cultured in both monolayer and 3D cell micromasses. ADSC cultures were daily stimulated for 36 min with ELF-EMFs or left unstimulated, and the progression of the differentiation process was evaluated by morphological analysis, extracellular matrix deposition, and gene expression profiling of chondrogenic markers. In both culturing conditions, stimulation with ELF-EMFs did not compromise cell viability but accelerated chondrogenesis by enhancing the secretion and deposition of extracellular matrix components at earlier time points in comparison to unstimulated cells. This study showed that, in an appropriate chondrogenic microenvironment, ELF-EMFs enhance chondrogenic differentiation and may be an important tool for supporting and accelerating the treatment of OA through autologous adipose stem cell therapy.

Molecular mechanism of miR-203a targeting Runx2 to regulate thiram induced-chondrocyte development

Thiram is a kind of organic compound, which is commonly used for sterilization, insecticidal and deodorization in daily life. Its toxicology has been broadly studied. Recently, more and more microRNAs have been shown to participate in the regulation of cartilage development. However, the potential mechanism by which microRNA regulates chondrocyte growth is still unclear. Our experiments have demonstrated that thiram can hamper chondrocytes development and cause a significant increase in miR-203a content in vitro and in vivo trials. miR-203a mimic significantly decrease in mRNA and protein expression of Wnt4, Runx2, COL2A1, β-catenin and ALP, and significantly enhance the mRNA and protein levels of GSK-3β. It has been observed that overexpression of miR-203a hindered chondrocytes development. In addition, Runx2 was confirmed to be a direct target of miR-203a by dual luciferase report gene assay. Transfection of si-Runx2 into chondrocytes reveals that significant downregulation of genes is associated with cartilage development. Overall, these results suggest that overexpression of miR-203a inhibits the expression of Runx2. These findings are conducive to elucidate the mechanism of chondrocytes dysplasia induced by thiram and provide new research ideas for the toxicology of thiram.

Inhibition of HOXD11 promotes cartilage degradation and induces osteoarthritis development

The 5'-HOXD genes are important for chondrogenesis in vertebrates, but their roles in osteoarthritis (OA) are still ambiguous. In our study, 5'-HOXD genes involvement contributing to cartilage degradation and OA was investigated. In bioinformatics analysis of 5'-HOXD genes, we obtained the GSE169077 data set related to OA in the GEO and analyzed DEGs using the GEO2R tool attached to the GEO. Then, we screened the mRNA levels of 5'-HOXD genes by quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR). We discovered that OA chondrocyte proliferation was inhibited, and apoptosis was increased. Moreover, it was discovered that SOX9 and COL2A1 were downregulated at mRNA and protein levels, while matrix metalloproteinases (MMPs) and a disintegrin-like and metalloproteinase with thrombospondin motifs (ADAMTSs) were upregulated. According to the results of differentially expressed genes (DEGs) and qRT-PCR, we evaluated the protein level of HOXD11 and found that the expression of HOXD11 was downregulated, reversed to MMPs and ADAMTSs but consistent with the cartilage-specific factors, SOX9 and COL2A1. In the lentivirus transfection experiments, HOXD11 overexpression reversed the effects in OA chondrocytes. In human OA articular cartilage, aberrant subchondral bone was formed in hematoxylin-eosin (H&E) and Safranin O and fast green (SOFG) staining results. Furthermore, according to immunohistochemistry findings, SOX9 and HOXD11 expression was inhibited. The results of this study established that HOXD11 was downregulated in OA cartilage and that overexpression of HOXD11 could prevent cartilage degradation in OA.

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

Identification of a second genetic alteration in patients with SHOX deficiency individuals: a potential explanation for phenotype variability

OBJECTIVE

Our study aimed to assess the impact of genetic modifiers on the significant variation in phenotype that is observed in individuals with SHOX deficiency, which is the most prevalent monogenic cause of short stature.

DESIGN AND METHODS

We performed a genetic analysis in 98 individuals from 48 families with SHOX deficiency with a target panel designed to capture the entire SHOX genomic region and 114 other genes that modulate growth and/or SHOX action. We prioritized rare potentially deleterious variants.

RESULTS

We did not identify potential deleterious variants in the promoter or intronic regions of the SHOX genomic locus. In contrast, we found eight heterozygous variants in 11 individuals from nine families in genes with a potential role as genetic modifiers. In addition to a previously described likely pathogenic (LP) variant in CYP26C1 observed in two families, we identified LP variants in PTHLH and ACAN, and variants of uncertain significance in NPR2, RUNX2, and TP53 in more affected individuals from families with SHOX deficiency. Families with a SHOX alteration restricted to the regulatory region had a higher prevalence of a second likely pathogenic variant (27%) than families with an alteration compromising the SHOX coding region (2.9%, P = .04).

CONCLUSION

In conclusion, variants in genes related to the growth plate have a potential role as genetic modifiers of the phenotype in individuals with SHOX deficiency. In individuals with SHOX alterations restricted to the regulatory region, a second alteration could be critical to determine the penetrance and expression of the phenotype.

Multiparity and Aging Impact Chondrogenic and Osteogenic Potential at Symphyseal Enthesis: New Insights into Interpubic Joint Remodeling

Pregnancy and childbirth cause adaptations to the birth canal to allow for delivery and fast recovery. To accommodate delivery through the birth canal, the pubic symphysis undergoes changes that lead to the interpubic ligament (IpL) and enthesis formation in primiparous mice. However, successive deliveries influence joint recovery. We aimed to understand tissue morphology and chondrogenic and osteogenic potential at symphyseal enthesis during pregnancy and postpartum in primiparous and multiparous senescent female mice. Morphological and molecular differences were found at the symphyseal enthesis among the study groups. Despite the apparent incapacity to restore cartilage in multiparous senescent animals, the symphyseal enthesis cells are active. However, these cells have reduced expression of chondrogenic and osteogenic markers and are immersed in densely packed collagen fibers contiguous to the persistent IpL. These findings may indicate alterations of key molecules in the progenitor cell population maintenance of the chondrocytic and osteogenic lineages at the symphyseal enthesis in multiparous senescent animals, possibly compromising the mouse joint histoarchitecture recovery. This sheds light on the distention of the birth canal and the pelvic floor that may play a role in pubic symphysis diastasis (PSD) and pelvic organ prolapse (POP), both in orthopedic and urogynecological practice in women.

Osteoarthritis: pathogenic signaling pathways and therapeutic targets

Osteoarthritis (OA) is a chronic degenerative joint disorder that leads to disability and affects more than 500 million population worldwide. OA was believed to be caused by the wearing and tearing of articular cartilage, but it is now more commonly referred to as a chronic whole-joint disorder that is initiated with biochemical and cellular alterations in the synovial joint tissues, which leads to the histological and structural changes of the joint and ends up with the whole tissue dysfunction. Currently, there is no cure for OA, partly due to a lack of comprehensive understanding of the pathological mechanism of the initiation and progression of the disease. Therefore, a better understanding of pathological signaling pathways and key molecules involved in OA pathogenesis is crucial for therapeutic target design and drug development. In this review, we first summarize the epidemiology of OA, including its prevalence, incidence and burdens, and OA risk factors. We then focus on the roles and regulation of the pathological signaling pathways, such as Wnt/β-catenin, NF-κB, focal adhesion, HIFs, TGFβ/ΒΜP and FGF signaling pathways, and key regulators AMPK, mTOR, and RUNX2 in the onset and development of OA. In addition, the roles of factors associated with OA, including MMPs, ADAMTS/ADAMs, and PRG4, are discussed in detail. Finally, we provide updates on the current clinical therapies and clinical trials of biological treatments and drugs for OA. Research advances in basic knowledge of articular cartilage biology and OA pathogenesis will have a significant impact and translational value in developing OA therapeutic strategies.

Downregulation of miR-30b-5p Facilitates Chondrocyte Hypertrophy and Apoptosis via Targeting Runx2 in Steroid-Induced Osteonecrosis of the Femoral Head

As a widely used steroid hormone medicine, glucocorticoids have the potential to cause steroid-induced osteonecrosis of the femoral head (SONFH) due to mass or long-term use. The non-coding RNA hypothesis posits that they may contribute to the destruction and dysfunction of cartilages as a possible etiology of SONFH. MiR-30b-5p was identified as a regulatory factor in cartilage degeneration caused by methylprednisolone (MPS) exposure in our study through cell transfection. The luciferase reporter assay confirmed that miR-30b-5p was downregulated and runt-related transcription factor 2 (Runx2) was mediated by miR-30b-5p. The nobly increased expression of matrix metallopeptidase 13 (MMP13) and type X collagen (Col10a1) as Runx2 downstream genes contributed to the hypertrophic differentiation of chondrocytes, and the efficiently upregulated level of matrix metallopeptidase 9 (MMP9) may trigger chondrocyte apoptosis with MPS treatments. The cell transfection experiment revealed that miR-30b-5p inhibited chondrocyte hypertrophy and suppressed MPS-induced apoptosis. As a result, our findings showed that miR-30b-5p modulated Runx2, MMP9, MMP13, and Col10a1 expression, thereby mediating chondrocyte hypertrophic differentiation and apoptosis during the SONFH process. These findings revealed the mechanistic relationship between non-coding RNA and SONFH, providing a comprehensive understanding of SONFH and other bone diseases.

Overexpression of transcription factor FoxA2 in the developing skeleton causes an enlargement of the cartilage hypertrophic zone, but it does not trigger ectopic differentiation in immature chondrocytes

We previously found that FoxA factors are necessary for chondrocyte differentiation. To investigate whether FoxA factors alone are sufficient to drive chondrocyte hypertrophy, we build a FoxA2 transgenic mouse in which FoxA2 cDNA is driven by a reiterated Tetracycline Response Element (TRE) and a minimal CMV promoter. This transgenic line was crossed with a col2CRE;Rosa26^rtTA/+ mouse line to generate col2CRE;Rosa26^rtTA/+;TgFoxA2^+/- mice for inducible expression of FoxA2 in cartilage using doxycycline treatment. Ectopic expression of FoxA2 in the developing skeleton reveals skeletal defects and shorter skeletal elements in E17.5 mice. The chondro-osseous border was frequently mis-shaped in mutant mice, with small islands of col.10+ hypertrophic cells extending in the metaphyseal bone. Even though overexpression of FoxA2 causes an accumulation of hypertrophic chondrocytes, it did not trigger ectopic hypertrophy in the immature chondrocytes. This suggests that FoxA2 may need transcriptional co-factors (such as Runx2), whose expression is restricted to the hypertrophic zone, and absent in the immature chondrocytes. To investigate a potential FoxA2/Runx2 interaction in immature chondrocytes versus hypertrophic cells, we separated these two subpopulations by FACS to obtain CD24⁺CD200⁺ hypertrophic chondrocytes and CD24⁺CD200^- immature chondrocytes and we ectopically expressed FoxA2 alone or in combination with Runx2 via lentiviral gene delivery. In CD24⁺CD200⁺ hypertrophic chondrocytes, FoxA2 enhanced the expression of chondrocyte hypertrophic markers collagen 10, MMP13, and alkaline phosphatase. In contrast, in the CD24⁺CD200^- immature chondrocytes, neither FoxA2 nor Runx2 overexpression could induce ectopic expression of hypertrophic markers MMP13, alkaline phosphatase, or PTH/PTHrP receptor. Overall these findings mirror our in vivo data, and suggest that induction of chondrocyte hypertrophy by FoxA2 may require other factors in addition to Runx2 (i.e., Hif2α, MEF2C, or perhaps unknown factors), whose expression/activity is rate-limiting in immature chondrocytes.

Effects of rAAV-mediated overexpression of bone morphogenetic protein 3 (BMP-3) on the chondrogenic fate of human bone marrow-derived mesenchymal stromal cells

Implantation of genetically modified chondrogenically competent human bone marrow-derived mesenchymal stromal cells (hMSCs) is an attractive strategy to improve cartilage repair. The goal of this study was to examine the potential benefits of transferring a sequence coding for the bone morphogenetic protein 3 (BMP-3) that modulates bone and cartilage formation, using recombinant adeno-associated virus (rAAV) vectors on the chondroreparative activities of hMSCs. Undifferentiated and chondrogenically induced primary human MSCs were treated with an rAAV-hBMP-3 construct to evaluate its effects on the proliferative, metabolic, and chondrogenic activities of the cells compared with control (reporter rAAV-lacZ vector) condition. Effective BMP-3 expression was noted both in undifferentiated and chondrogenically differentiated cells in the presence of rAAV-hBMP-3 relative to rAAV-lacZ, stimulating cell proliferation and extracellular matrix (proteoglycans, type-II collagen) deposition together with higher levels of chondrogenic SOX9 expression. rAAV-hBMP-3 also advantageously decreased terminal differentiation, hypertrophy, and osteogenesis (type-I/-X collagen and alkaline phosphatase expression), with reduced levels of osteoblast-related RUNX-2 transcription factor and β-catenin (osteodifferentiation mediator) and enhanced PTHrP expression (inhibitor of hypertrophic maturation, calcification, and bone formation). This study shows the advantage of modifying hMSCs with rAAV-hBMP-3 to trigger adapted chondroreparative activities as a source of improved cells for transplantation protocols in cartilage defects.

Multiple roles of Runt-related transcription factor-2 in tooth eruption: bone formation and resorption

OBJECTIVE

The aim was to provide a comprehensive review of the current knowledge of the multiple roles of Runt-related transcription factor-2 (RUNX2) in regulating tooth eruption, focusing on the molecular mechanisms regarding tooth eruption mediated by RUNX2.

DESIGN

Relevant literatures in PubMed, Medline, and Scopus database were searched, and a narrative review was performed. The multiple roles of RUNX2 in regulating tooth eruption was reviewed and discussed.

RESULTS

Aberrant RUNX2 expression leads to disturbed or failed tooth eruption. Tooth eruption involves both the process of bone formation and bone resorption. RUNX2 promotes osteogenesis around the radicular portion of the dental follicle that provides the biological force for tooth eruption through inducing the expression of osteogenesis-related genes in dental follicle cells/osteoblasts. On the other hand, through indirect and direct pathways, RUNX2 regulates osteoclastogenesis and the formation of the eruption pathway.

CONCLUSION

RUNX2 exerts a pivotal and complex influence in regulating tooth eruption. This review provides a better understanding of the function of RUNX2 in tooth eruption, which is beneficial to illuminate the precise molecular mechanism of osteogenesis and bone resorption, aiding the development of effective therapy for the failure of tooth eruption.

Species-specific sensitivity to TGFβ signaling and changes to the Mmp13 promoter underlie avian jaw development and evolution

Precise developmental control of jaw length is critical for survival, but underlying molecular mechanisms remain poorly understood. The jaw skeleton arises from neural crest mesenchyme (NCM), and we previously demonstrated that these progenitor cells express more bone-resorbing enzymes including Matrix metalloproteinase 13 (Mmp13) when they generate shorter jaws in quail embryos versus longer jaws in duck. Moreover, if we inhibit bone resorption or Mmp13, we can increase jaw length. In the current study, we uncover mechanisms establishing species-specific levels of Mmp13 and bone resorption. Quail show greater activation of and sensitivity to transforming growth factor beta (TGFβ) signaling than duck; where intracellular mediators like SMADs and targets like Runt-related transcription factor 2 (Runx2), which bind Mmp13, become elevated. Inhibiting TGFβ signaling decreases bone resorption, and overexpressing Mmp13 in NCM shortens the duck lower jaw. To elucidate the basis for this differential regulation, we examine the Mmp13 promoter. We discover a SMAD-binding element and single nucleotide polymorphisms (SNPs) near a RUNX2-binding element that distinguish quail from duck. Altering the SMAD site and switching the SNPs abolish TGFβ sensitivity in the quail Mmp13 promoter but make the duck promoter responsive. Thus, differential regulation of TGFβ signaling and Mmp13 promoter structure underlie avian jaw development and evolution.

Effects of Separate and Combined Exposure of Cadmium and Lead on the Endochondral Ossification in Bufo gargarizans

Cadmium (Cd) and lead (Pb) are ubiquitous in aquatic environments and most studies have examined the potential effects of Cd or Pb alone on aquatic organisms. In the present study, chronic effects of Cd and Pb, alone and in combination, on Bufo gargarizans were investigated by exposing embryos to these contaminants throughout metamorphosis. Significant reductions in body mass and snout-to-vent length were observed in B. gargarizans at Gosner stage 42 (Gs 42) and Gs 46 exposed to a Cd/Pb mixture. Single and combined exposure with Cd and Pb induced histological alterations of the thyroid gland characterized by reduced colloid area and thickness of epithelial cells. There was a significant decrease in the maximum jump distance of froglets exposed to Cd alone and the Cd/Pb mixture, and the jumping capacity showed a positive correlation with hind limb length and tibia/fibula. Moreover, single metals and their mixture induced reduction of endochondral bone formation in B. gargarizans. Transcriptomic and real-time quantitative polymerase chain reaction results showed that genes involved in skeletal ossification (TRα, TRβ, Dio2, Dio3, MMP9, MMP13, Runx1, Runx2, and Runx3) were transcriptionally dysregulated by Cd and Pb exposure alone or in combination. Our results suggested that despite the low concentration tested, the Cd/Pb mixture induced more severe impacts on B. gargarizans. In addition, the Cd/Pb mixture might reduce chances of survival for B. gargarizans froglets by decreasing size at metamorphosis, impaired skeletal ossification, and reduction in jumping ability, which might result from dysregulation of genes involved in thyroid hormone action and endochondral ossification. The findings obtained could add a new dimension to understanding of the mechanisms underpinning skeletal ossification response to heavy metals in amphibians. Environ Toxicol Chem 2022;41:1228-1245. © 2022 SETAC.

Identification of a familial cleidocranial dysplasia with a novel RUNX2 mutation and establishment of patient-derived induced pluripotent stem cells

Cleidocranial dysplasia (CCD) is an autosomal dominant hereditary disease associated with the gene RUNX2. Disease-specific induced pluripotent stem cells (iPSCs) have emerged as a useful resource to further study human hereditary diseases such as CCD. In this study, we identified a novel CCD-specific RUNX2 mutation and established iPSCs with this mutation. Biopsies were obtained from familial CCD patients and mutation analyses were performed through Sanger sequencing and next generation sequencing. CCD-specific human iPSCs (CCD-hiPSCs) were established and maintained under completely defined serum, feeder, and integration-free condition using a non-integrating replication-defective Sendai virus vector. We identified the novel mutation RUNX2_c.371C>G and successfully established CCD-hiPSCs. The CCD-hiPSCs inherited the same mutation, possessed pluripotency, and showed the ability to differentiate the three germ layers. We concluded that RUNX2_c.371C>G was likely pathogenic because our results, derived from next generation sequencing, are supported by actual clinical evidence, familial tracing, and genetic data. Thus, we concluded that hiPSCs with a novel CCD-specific RUNX2 mutation are viable as a resource for future studies on CCD.

Runt-related transcription factor-2 (Runx2) is required for bone matrix protein gene expression in committed osteoblasts in mice

Runt-related transcription factor-2 (Runx2) is an essential transcription factor for osteoblast differentiation. However, its functions after the commitment into osteoblasts are controversial and remain to be clarified. We generated enhanced green fluorescent protein (EGFP)-Cre transgenic mice driven by the 2.3-kilobase (kb) Col1a1 promoter, and Runx2 was deleted in osteoblasts and odontoblasts in Runx2^fl/flCre mice. The sutures and fontanelles were more widely opened in Runx2^fl/flCre newborns than in Runx2^fl/fl newborns. Runx2^fl/flCre mice exhibited dwarfism with shorter incisors and 37% had irregularly aligned incisors. The volume of trabecular bone in femurs and vertebrae and their bone mineral density (BMD), in addition to the cortical thickness and BMD were reduced in Runx2^fl/flCre mice compared with Runx2^fl/fl mice in both sexes. The bone formation of both trabecular and cortical bone, osteoblast number, osteoclast surface, osteoblast proliferation, and the serum levels of procollagen type 1 N-terminal propeptide (P1NP), tartrate-resistant acid phosphatase 5b (TRAP5b), and C-terminal cross-linked telopeptide of type 1 collagen (CTX1) were reduced in Runx2^fl/flCre mice. The expression of major bone matrix protein genes, including Col1a1, Col1a2, Spp1, Ibsp, and Bglap&Bglap2, and of Tnfsf11 was lower in Runx2^fl/flCre mice than in Runx2^fl/fl mice. The expression of Runx2 target genes, including Ihh, Fgfr1, Fgfr2, Fgfr3, Tcf7, Wnt10b, Pth1r, Sp7, and Dlx5, was also reduced. Osteoblasts in Runx2^fl/fl mice were cuboidal and contained abundant type I collagen α1 (Col1a1), whereas those in Runx2^fl/flCre mice were deflated and contained a small amount of Col1a1. Runx2 activated the reporter activity of the 2.3-kb Col1a1 promoter and bound the region around the Col1a1 transcription start site. The deletion of Runx2 by Cre-expressing adenovirus in Runx2^fl/fl primary osteoblasts impaired osteoblast differentiation and the expression of genes encoding major bone matrix proteins, and osteoclastogenesis was inhibited due to the reduction of Tnfsf11 expression in the osteoblasts. This study demonstrated that Runx2 is required for the expression of the major bone matrix protein genes and Tnfsf11 after commitment into osteoblasts in mice. © 2021 American Society for Bone and Mineral Research (ASBMR).

miR-140-3p regulates the osteogenic differentiation ability of bone marrow mesenchymal stem cells by targeting spred2-mediated autophagy

Understanding the function and regulatory mechanism of miR-140-3p on the osteogenic differentiation of bone mesenchymal stem cells (BMSCs). Alizarin Red staining, Alkaline phosphatase (ALP) staining, and ALP activity were used to detect the ability osteogenic differentiation. miR-140-3p or Spred2 overexpression into BMSCs using lentiviral vectors and the result were analyzed by Reverse transcription quantitative polymerase chain reaction (RT-qPCR). The relation between miR-140-3p and Spred2 was examined by luciferase reporter assay. CCK8 assay was used to detect the proliferation of BMSCs. RT-qPCR and Western blot analysis were both used to detect altered gene and protein in osteogenic differentiation of BMSCs, respectively. The BMSCs which were induced for 21 days were analyzed by Alizarin Red staining, (ALP) staining and ALP activity. RT-qPCR analysis showed that overexpressed miR-140-3p promotes osteogenic differentiation. Western blots results indicated that the overexpression of Spred2 suppressed miR-140-3p. Luciferase reporter assay indicated that Spred2 can integrate with miR-140-3p directly. Meanwhile, the protein level of ALP, OCN, and Runx2, the markers of chondrogenesis, was increased when miR-140-3p increased or Spred2 overexpressed in the osteoinductive medium applied to the BMSCs. Our study demonstrated the association between miR-140-3p and Spred2 in osteogenic differentiation of BMSCs for the first time. Furthermore, our detections also revealed that Spred2-induced autophagic signaling accelerates the progress of osteogenic differentiation ability of BMSCs.

RUNX2 regulates leukemic cell metabolism and chemotaxis in high-risk T cell acute lymphoblastic leukemia

T cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic malignancy with inferior outcome compared with that of B cell ALL. Here, we show that Runt-related transcription factor 2 (RUNX2) was upregulated in high-risk T-ALL with KMT2A rearrangements (KMT2A-R) or an immature immunophenotype. In KMT2A-R cells, we identified RUNX2 as a direct target of the KMT2A chimeras, where it reciprocally bound the KMT2A promoter, establishing a regulatory feed-forward mechanism. Notably, RUNX2 was required for survival of immature and KMT2A-R T-ALL cells in vitro and in vivo. We report direct transcriptional regulation of CXCR4 signaling by RUNX2, thereby promoting chemotaxis, adhesion, and homing to medullary and extramedullary sites. RUNX2 enabled these energy-demanding processes by increasing metabolic activity in T-ALL cells through positive regulation of both glycolysis and oxidative phosphorylation. Concurrently, RUNX2 upregulation increased mitochondrial dynamics and biogenesis in T-ALL cells. Finally, as a proof of concept, we demonstrate that immature and KMT2A-R T-ALL cells were vulnerable to pharmacological targeting of the interaction between RUNX2 and its cofactor CBFβ. In conclusion, we show that RUNX2 acts as a dependency factor in high-risk subtypes of human T-ALL through concomitant regulation of tumor metabolism and leukemic cell migration.

Dmrt2 promotes transition of endochondral bone formation by linking Sox9 and Runx2

Endochondral bone formation is fundamental for skeletal development. During this process, chondrocytes undergo multiple steps of differentiation and coordinated transition from a proliferating to a hypertrophic stage, which is critical to advance skeletal development. Here, we identified the transcription factor Dmrt2 (double-sex and mab-3 related transcription factor 2) as a Sox9-inducible gene that promotes chondrocyte hypertrophy in pre-hypertrophic chondrocytes. Epigenetic analysis further demonstrated that Sox9 regulates Dmrt2 expression through an active enhancer located 18 kb upstream of the Dmrt2 gene and that this enhancer's chromatin status is progressively activated through chondrocyte differentiation. Dmrt2-knockout mice exhibited a dwarf phenotype with delayed initiation of chondrocyte hypertrophy. Dmrt2 augmented hypertrophic chondrocyte gene expression including Ihh through physical and functional interaction with Runx2. Furthermore, Dmrt2 deficiency reduced Runx2-dependent Ihh expression. Our findings suggest that Dmrt2 is critical for sequential chondrocyte differentiation during endochondral bone formation and coordinates the transcriptional network between Sox9 and Runx2.

Overview of Skeletal Development

Development of cartilage and bone, the core components of the mouse skeletal system, depends on coordinated proliferation and differentiation of skeletogenic cells, including chondrocytes and osteoblasts. These cells differentiate from common progenitor cells originating in the mesoderm and neural crest. Multiple signaling pathways and transcription factors tightly regulate differentiation and proliferation of skeletal cells. In this chapter, we overview the process of mouse skeletal development and discuss major regulators of skeletal cells at each developmental stage.

Tissue Engineering of Cartilage Using a Random Positioning Machine

Articular cartilage is a skeletal tissue of avascular nature and limited self-repair capacity. Cartilage-degenerative diseases, such as osteoarthritis (OA), are difficult to treat and often necessitate joint replacement surgery. Cartilage is a tough but flexible material and relatively easy to damage. It is, therefore, of high interest to develop methods allowing chondrocytes to recolonize, to rebuild the cartilage and to restore joint functionality. Here we studied the in vitro production of cartilage-like tissue using human articular chondrocytes exposed to the Random Positioning Machine (RPM), a device to simulate certain aspects of microgravity on Earth. To screen early adoption reactions of chondrocytes exposed to the RPM, we performed quantitative real-time PCR analyses after 24 h on chondrocytes cultured in DMEM/F-12. A significant up-regulation in the gene expression of IL6, RUNX2, RUNX3, SPP1, SOX6, SOX9, and MMP13 was detected, while the levels of IL8, ACAN, PRG4, ITGB1, TGFB1, COL1A1, COL2A1, COL10A1, SOD3, SOX5, MMP1, and MMP2 mRNAs remained unchanged. The STRING (Search Tool for the Retrieval of Interacting Genes/Proteins) analysis demonstrated among others the importance of these differentially regulated genes for cartilage formation. Chondrocytes grown in DMEM/F-12 medium produced three-dimensional (3D) spheroids after five days without the addition of scaffolds. On day 28, the produced tissue constructs reached up to 2 mm in diameter. Using specific chondrocyte growth medium, similar results were achieved within 14 days. Spheroids from both types of culture media showed the typical cartilage morphology with aggrecan positivity. Intermediate filaments form clusters under RPM conditions as detected by vimentin staining after 7 d and 14 d. Larger meshes appear in the network in 28-day samples. Furthermore, they were able to form a confluent chondrocyte monolayer after being transferred back into cell culture flasks in 1 g conditions showing their suitability for transplantation into joints. Our results demonstrate that the cultivation medium has a direct influence on the velocity of tissue formation and tissue composition. The spheroids show properties that make them interesting candidates for cellular cartilage regeneration approaches in trauma and OA therapy.

Runx2 is essential for the transdifferentiation of chondrocytes into osteoblasts

Chondrocytes proliferate and mature into hypertrophic chondrocytes. Vascular invasion into the cartilage occurs in the terminal hypertrophic chondrocyte layer, and terminal hypertrophic chondrocytes die by apoptosis or transdifferentiate into osteoblasts. Runx2 is essential for osteoblast differentiation and chondrocyte maturation. Runx2-deficient mice are composed of cartilaginous skeletons and lack the vascular invasion into the cartilage. However, the requirement of Runx2 in the vascular invasion into the cartilage, mechanism of chondrocyte transdifferentiation to osteoblasts, and its significance in bone development remain to be elucidated. To investigate these points, we generated Runx2fl/flCre mice, in which Runx2 was deleted in hypertrophic chondrocytes using Col10a1 Cre. Vascular invasion into the cartilage was similarly observed in Runx2fl/fl and Runx2fl/flCre mice. Vegfa expression was reduced in the terminal hypertrophic chondrocytes in Runx2fl/flCre mice, but Vegfa was strongly expressed in osteoblasts in the bone collar, suggesting that Vegfa expression in bone collar osteoblasts is sufficient for vascular invasion into the cartilage. The apoptosis of terminal hypertrophic chondrocytes was increased and their transdifferentiation was interrupted in Runx2fl/flCre mice, leading to lack of primary spongiosa and osteoblasts in the region at E16.5. The osteoblasts appeared in this region at E17.5 in the absence of transdifferentiation, and the number of osteoblasts and the formation of primary spongiosa, but not secondary spongiosa, reached to levels similar those in Runx2fl/fl mice at birth. The bone structure and volume and all bone histomophometric parameters were similar between Runx2fl/fl and Runx2fl/flCre mice after 6 weeks of age. These findings indicate that Runx2 expression in terminal hypertrophic chondrocytes is not required for vascular invasion into the cartilage, but is for their survival and transdifferentiation into osteoblasts, and that the transdifferentiation is necessary for trabecular bone formation in embryonic and neonatal stages, but not for acquiring normal bone structure and volume in young and adult mice.

Lineage-specific differentiation of osteogenic progenitors from pluripotent stem cells reveals the FGF1-RUNX2 association in neural crest-derived osteoprogenitors

Human pluripotent stem cells (hPSCs) can provide a platform to model bone organogenesis and disease. To reflect the developmental process of the human skeleton, hPSC differentiation methods should include osteogenic progenitors (OPs) arising from three distinct embryonic lineages: the paraxial mesoderm, lateral plate mesoderm, and neural crest. Although OP differentiation protocols have been developed, the lineage from which they are derived, as well as characterization of their genetic and molecular differences, has not been well reported. Therefore, to generate lineage-specific OPs from human embryonic stem cells and human induced pluripotent stem cells, we employed stepwise differentiation of paraxial mesoderm-like cells, lateral plate mesoderm-like cells, and neural crest-like cells toward their respective OP subpopulation. Successful differentiation, confirmed through gene expression and in vivo assays, permitted the identification of transcriptomic signatures of all three cell populations. We also report, for the first time, high FGF1 levels in neural crest-derived OPs-a notable finding given the critical role of fibroblast growth factors (FGFs) in osteogenesis and mineral homeostasis. Our results indicate that FGF1 influences RUNX2 levels, with concomitant changes in ERK1/2 signaling. Overall, our study further validates hPSCs' power to model bone development and disease and reveals new, potentially important pathways influencing these processes.

New Insights Into Cranial Synchondrosis Development: A Mini Review

The synchondroses formed via endochondral ossification in the cranial base are an important growth center for the neurocranium. Abnormalities in the synchondroses affect cranial base elongation and the development of adjacent regions, including the craniofacial bones. In the central region of the cranial base, there are two synchondroses present—the intersphenoid synchondrosis and the spheno-occipital synchondrosis. These synchondroses consist of mirror image bipolar growth plates. The cross-talk of several signaling pathways, including the parathyroid hormone-like hormone (PTHLH)/parathyroid hormone-related protein (PTHrP), Indian hedgehog (Ihh), Wnt/β-catenin, and fibroblast growth factor (FGF) pathways, as well as regulation by cilium assembly and the transcription factors encoded by the RUNX2, SIX1, SIX2, SIX4, and TBX1 genes, play critical roles in synchondrosis development. Deletions or activation of these gene products in mice causes the abnormal ossification of cranial synchondrosis and skeletal elements. Gene disruption leads to both similar and markedly different abnormalities in the development of intersphenoid synchondrosis and spheno-occipital synchondrosis, as well as in the phenotypes of synchondroses and skeletal bones. This paper reviews the development of cranial synchondroses, along with its regulation by the signaling pathways and transcription factors, highlighting the differences between intersphenoid synchondrosis and spheno-occipital synchondrosis.

Human pluripotent stem cell-derived chondroprogenitors for cartilage tissue engineering

The cartilage of joints, such as meniscus and articular cartilage, is normally long lasting (i.e., permanent). However, once damaged, especially in large animals and humans, joint cartilage is not spontaneously repaired. Compensating the lack of repair activity by supplying cartilage-(re)forming cells, such as chondrocytes or mesenchymal stromal cells, or by transplanting a piece of normal cartilage, has been the basis of therapy for biological restoration of damaged joint cartilage. Unfortunately, current biological therapies face problems on a number of fronts. The joint cartilage is generated de novo from a specialized cell type, termed a 'joint progenitor' or 'interzone cell' during embryogenesis. Therefore, embryonic chondroprogenitors that mimic the property of joint progenitors might be the best type of cell for regenerating joint cartilage in the adult. Pluripotent stem cells (PSCs) are expected to differentiate in culture into any somatic cell type through processes that mimic embryogenesis, making human (h)PSCs a promising source of embryonic chondroprogenitors. The major research goals toward the clinical application of PSCs in joint cartilage regeneration are to (1) efficiently generate lineage-specific chondroprogenitors from hPSCs, (2) expand the chondroprogenitors to the number needed for therapy without loss of their chondrogenic activity, and (3) direct the in vivo or in vitro differentiation of the chondroprogenitors to articular or meniscal (i.e., permanent) chondrocytes rather than growth plate (i.e., transient) chondrocytes. This review is aimed at providing the current state of research toward meeting these goals. We also include our recent achievement of successful generation of "permanent-like" cartilage from long-term expandable, hPSC-derived ectomesenchymal chondroprogenitors.

Runx2 silencing promotes adipogenesis via down-regulation of DLK1 in chondrogenic differentiating MSCs

BACKGROUND

For cartilage regeneration, stem cells are a promising cell source; however, even the advances made in the differentiation of stem cells into precursor-differentiated cartilage cells have not been successful with respect to reprograming these cells to achieve complete differentiation and fully functioning cells until now. Previous findings suggest that Runx2 plays a major role in chondrocyte differentiation and maturation. Although targeting Runx2 has enhanced some chondrocyte properties, the adipogenic lineage shift has eventually occurred in these cells. The present study mainly aimed to reveal the mechanism of this adipogenesis.

METHODS

To create inducible artificial shRNA-miR expressing vectors, the designed short hairpin RNAs (shRNAs) were inserted into the pri-mir-30 backbone, cloned into lentiviral pLVET-Tet-on, and transducted into mesenchymal stem cells (MSCs). Runx2 gene was silenced in MSCs either for 1 week or 4 weeks and cultured in the chondrogenic medium. At days 7, 14 and 28, cells were harvested, and chondrogenesis, adipogenesis and hypertrophic states were examined using histochemical staining and a real-time polymerase chain reaction assay.

RESULTS

The results showed that the designed shRNA-miR effectively targeted Runx2 in mRNA and protein levels. Chondrogenic markers were up-regulated in constantly silenced Runx2 group; however, adipogenic markers and fat droplets appeared gradually. DLK1 gene was also significantly down-regulated in this group, and overexpression of DLK1 abrogated adipogenesis in the Runx2 targeted group.

CONCLUSIONS

Based on these results, it can be concluded that DLK1 is responsible for the lineage shift in Runx2 targeted chondrogenic differentiating MSCs.

TBX1 Regulates Chondrocyte Maturation in the Spheno-occipital Synchondrosis

The synchondrosis in the cranial base is an important growth center for the craniofacial region. Abnormalities in the synchondroses affect the development of adjacent regions, including the craniofacial skeleton. Here, we report that the transcription factor TBX1, the candidate gene for DiGeorge syndrome, is expressed in mesoderm-derived chondrocytes and plays an essential and specific role in spheno-occipital synchondrosis development by inhibiting the expression of genes involved in chondrocyte hypertrophy and osteogenesis. In Tbx1-deficient mice, the spheno-occipital synchondrosis was completely mineralized at birth. TBX1 interacts with RUNX2, a master molecule of osteoblastogenesis and a regulator of chondrocyte maturation, and suppresses its transcriptional activity. Indeed, deleting Tbx1 triggers accelerated mineralization due to accelerated chondrocyte differentiation, which is associated with ectopic expression of downstream targets of RUNX2 in the spheno-occipital synchondrosis. These findings reveal that TBX1 acts as a regulator of chondrocyte maturation and osteogenesis during the spheno-occipital synchondrosis development. Thus, the tight regulation of endochondral ossification by TBX1 is crucial for the normal progression of chondrocyte differentiation in the spheno-occipital synchondrosis.

Expression of a Constitutively Active Form of Hck in Chondrocytes Activates Wnt and Hedgehog Signaling Pathways, and Induces Chondrocyte Proliferation in Mice

Runx2 is required for chondrocyte proliferation and maturation. In the search of Runx2 target genes in chondrocytes, we found that Runx2 up-regulated the expression of hematopoietic cell kinase (Hck), which is a member of the Src tyrosine kinase family, in chondrocytes, that Hck expression was high in cartilaginous limb skeletons of wild-type mice but low in those of Runx2-/- mice, and that Runx2 bound the promoter region of Hck. To investigate the functions of Hck in chondrocytes, transgenic mice expressing a constitutively active form of Hck (HckCA) were generated using the Col2a1 promoter/enhancer. The hind limb skeletons were fused, the tibia became a large, round mass, and the growth plate was markedly disorganized. Chondrocyte maturation was delayed until E16.5 but accelerated thereafter. BrdU-labeled, but not terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL)-positive, chondrocytes were increased. Furthermore, Hck knock-down reduced the proliferation of primary chondrocytes. In microarray and real-time RT-PCR analyses using hind limb RNA from HckCA transgenic mice, the expression of Wnt (Wnt10b, Tcf7, Lef1, Dkk1) and hedgehog (Ihh, Ptch1, and Gli1) signaling pathway genes was upregulated. These findings indicated that Hck, whose expression is regulated by Runx2, is highly expressed in chondrocytes, and that HckCA activates Wnt and hedgehog signaling pathways, and promotes chondrocyte proliferation without increasing apoptosis.

Antxr1, Which is a Target of Runx2, Regulates Chondrocyte Proliferation and Apoptosis

Antxr1/Tem8 is highly expressed in tumor endothelial cells and is a receptor for anthrax toxin. Mutation of Antxr1 causes GAPO syndrome, which is characterized by growth retardation, alopecia, pseudo-anodontia, and optic atrophy. However, the mechanism underlying the growth retardation remains to be clarified. Runx2 is essential for osteoblast differentiation and chondrocyte maturation and regulates chondrocyte proliferation through Ihh induction. In the search of Runx2 target genes in chondrocytes, we found that Antxr1 expression is upregulated by Runx2. Antxr1 was highly expressed in cartilaginous tissues and was directly regulated by Runx2. In skeletal development, the process of endochondral ossification proceeded similarly in wild-type and Antxr1-/- mice. However, the limbs of Antxr1-/- mice were shorter than those of wild-type mice from embryonic day 16.5 due to the reduced chondrocyte proliferation. Chondrocyte-specific Antxr1 transgenic mice exhibited shortened limbs, although the process of endochondral ossification proceeded as in wild-type mice. BrdU-uptake and apoptosis were both increased in chondrocytes, and the apoptosis-high regions were mineralized. These findings indicated that Antxr1, of which the expression is regulated by Runx2, plays an important role in chondrocyte proliferation and that overexpression of Antxr1 causes chondrocyte apoptosis accompanied by matrix mineralization.

Significance of MEF2C and RUNX3 Regulation for Endochondral Differentiation of Human Mesenchymal Progenitor Cells

Guiding progenitor cell development between chondral versus endochondral pathways is still an unachieved task of cartilage neogenesis, and human mesenchymal progenitor cell (MPC) chondrogenesis is considered as a valuable model to better understand hypertrophic development of chondrocytes. Transcription factors Runx2, Runx3, and Mef2c play prominent roles for chondrocyte hypertrophy during mouse development, but little is known on the importance of these key fate-determining factors for endochondral development of human MPCs. The aim of this study was to unravel the regulation of RUNX2, RUNX3, and MEF2C during MPC chondrogenesis, the pathways driving their expression, and the downstream hypertrophic targets affected by their regulation. RUNX2, RUNX3, and MEF2C gene expression was differentially regulated during chondrogenesis of MPCs, but remained low and unregulated when non-hypertrophic articular chondrocytes were differentiated under the same conditions. RUNX3 and MEF2C mRNA and protein levels rose in parallel to hypertrophic marker upregulation, but surprisingly, RUNX2 gene expression changed only by trend and RUNX2 protein remained undetectable. While RUNX3 expression was driven by TGF-β and BMP signaling, MEF2C responded to WNT-, BMP-, and Hedgehog-pathway inhibition. MEF2C but not RUNX3 levels correlated significantly with COL10A1, IHH, and IBSP gene expression when hypertrophy was attenuated. IBSP was a downstream target of RUNX3 and MEF2C but not RUNX2 in SAOS-2 cells, underlining the capacity of RUNX3 and MEF2C to stimulate osteogenic marker expression in human cells. Conclusively, RUNX3 and MEF2C appeared more important than RUNX2 for human endochondral MPC chondrogenesis. Pathways altering the speed of chondrogenesis (FGF, TGF-β, BMP) affected RUNX2 or RUNX3, while pathways changing hypertrophy (WNT, PTHrP/HH) regulated mainly MEF2C. Taken together, reduction of MEF2C levels is a new goal to shift human cartilage neogenesis toward the chondral pathway.

Role of thyroid hormones in craniofacial development

The development of the craniofacial skeleton relies on complex temporospatial organization of diverse cell types by key signalling molecules. Even minor disruptions to these processes can result in deleterious consequences for the structure and function of the skull. Thyroid hormone deficiency causes delayed craniofacial and tooth development, dysplastic facial features and delayed development of the ossicles in the middle ear. Thyroid hormone excess, by contrast, accelerates development of the skull and, in severe cases, might lead to craniosynostosis with neurological sequelae and facial hypoplasia. The pathogenesis of these important abnormalities remains poorly understood and underinvestigated. The orchestration of craniofacial development and regulation of suture and synchondrosis growth is dependent on several critical signalling pathways. The underlying mechanisms by which these key pathways regulate craniofacial growth and maturation are largely unclear, but studies of single-gene disorders resulting in craniofacial malformations have identified a number of critical signalling molecules and receptors. The craniofacial consequences resulting from gain-of-function and loss-of-function mutations affecting insulin-like growth factor 1, fibroblast growth factor receptor and WNT signalling are similar to the effects of altered thyroid status and mutations affecting thyroid hormone action, suggesting that these critical pathways interact in the regulation of craniofacial development.

Molecular Mechanism of Runx2-Dependent Bone Development

Runx2 is an essential transcription factor for skeletal development. It is expressed in multipotent mesenchymal cells, osteoblast-lineage cells, and chondrocytes. Runx2 plays a major role in chondrocyte maturation, and Runx3 is partly involved. Runx2 regulates chondrocyte proliferation by directly regulating Ihh expression. It also determines whether chondrocytes become those that form transient cartilage or permanent cartilage, and functions in the pathogenesis of osteoarthritis. Runx2 is essential for osteoblast differentiation and is required for the proliferation of osteoprogenitors. Ihh is required for Runx2 expression in osteoprogenitors, and hedgehog signaling and Runx2 induce the differentiation of osteoprogenitors to preosteoblasts in endochondral bone. Runx2 induces Sp7 expression, and Runx2, Sp7, and canonical Wnt signaling are required for the differentiation of preosteoblasts to immature osteoblasts. It also induces the proliferation of osteoprogenitors by directly regulating the expression of Fgfr2 and Fgfr3. Furthermore, Runx2 induces the proliferation of mesenchymal cells and their commitment into osteoblast-lineage cells through the induction of hedgehog (Gli1, Ptch1, Ihh), Fgf (Fgfr2, Fgfr3), Wnt (Tcf7, Wnt10b), and Pthlh (Pth1r) signaling pathway gene expression in calvaria, and more than a half-dosage of Runx2 is required for their expression. This is a major cause of cleidocranial dysplasia, which is caused by heterozygous mutation of RUNX2. Cbfb, which is a co-transcription factor that forms a heterodimer with Runx2, enhances DNA binding of Runx2 and stabilizes Runx2 protein by inhibiting its ubiquitination. Thus, Runx2/Cbfb regulates the proliferation and differentiation of chondrocytes and osteoblast-lineage cells by activating multiple signaling pathways and via their reciprocal regulation.

Evolutionary repression of chondrogenic genes in the vertebrate osteoblast

Gene expression in extant animals might reveal how skeletal cells have evolved over the past 500 million years. The cells that make up cartilage (chondrocytes) and bone (osteoblasts) express many of the same genes, but they also have important molecular differences that allow us to distinguish them as separate cell types. For example, traditional studies of later-diverged vertebrates, such as mouse and chick, defined the genes Col2a1 and sex-determining region Y-box 9 as cartilage-specific. However, recent studies have shown that osteoblasts of earlier-diverged vertebrates, such as frog, gar, and zebrafish, express these 'chondrogenic' markers. In this review, we examine the resulting hypothesis that chondrogenic gene expression became repressed in osteoblasts over evolutionary time. The amphibian is an underexplored skeletal model that is uniquely positioned to address this hypothesis, especially given that it diverged when life transitioned from water to land. Given the relationship between phylogeny and ontogeny, a novel discovery for skeletal cell evolution might bolster our understanding of skeletal cell development.

RUNX transcription factors: orchestrators of development

RUNX transcription factors orchestrate many different aspects of biology, including basic cellular and developmental processes, stem cell biology and tumorigenesis. In this Primer, we introduce the molecular hallmarks of the three mammalian RUNX genes, RUNX1, RUNX2 and RUNX3, and discuss the regulation of their activities and their mechanisms of action. We then review their crucial roles in the specification and maintenance of a wide array of tissues during embryonic development and adult homeostasis.

TGFβ/BMP Signaling Pathway in Cartilage Homeostasis

Cartilage homeostasis is governed by articular chondrocytes via their ability to modulate extracellular matrix production and degradation. In turn, chondrocyte activity is regulated by growth factors such as those of the transforming growth factor β (TGFβ) family. Members of this family include the TGFβs, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs). Signaling by this protein family uniquely activates SMAD-dependent signaling and transcription but also activates SMAD-independent signaling via MAPKs such as ERK and TAK1. This review will address the pivotal role of the TGFβ family in cartilage biology by listing several TGFβ family members and describing their signaling and importance for cartilage maintenance. In addition, it is discussed how (pathological) processes such as aging, mechanical stress, and inflammation contribute to altered TGFβ family signaling, leading to disturbed cartilage metabolism and disease.

Maintenance of SOX9 stability and ECM homeostasis by selenium-sensitive PRMT5 in cartilage

OBJECTIVES

Selenium (Se) plays pivotal roles in maintaining optimal health. Nevertheless, how Se influences the metabolism of extracellular matrix (ECM) in cartilage remains unclear. The aim of the present study was to observe protein dimethylation by certain Se-sensitive PRMT and to elucidate its effects on the key transcriptional factor in cartilage.

METHODS

We observed the expression of selenoproteins and markers of ECM metabolism in chondrocytes and articular cartilage of the rats under Se-deficiency by RT-qPCR, immunoblotting and immunohistochemistry. Then, we analyzed the expression of total dimethylated protein by using specific antibody under different Se statuses. After Se sensitive PRMT was identified, we used siRNA or PRMT inhibitor or stably overexpressing vector to intervene in the PRMT expression and identified the key transcriptional factor. Co-immunoprecipitation was applied to verify the interaction between PRMT and the key transcriptional factor. Finally, we measured the half-life time of the key transcriptional factor by immunoblotting after cycloheximide treatment.

RESULTS

In chondrocytes and cartilage of the rats with Se deficiency, we found an aberrant metabolism manifesting decreased expression of Col2a1 and increased expression of Mmp-3. Then, we identified that PRMT5 was the unique type II PRMT, sensitive to Se status. PRMT5 upregulation led to the increased COL2A1 expression but decreased MMP-3 expression in chondrocytes. Furthermore, we revealed that PRMT5 improved SOX9 stability by dimethylating the protein, which contributed to maintain the matrix metabolic homeostasis of the chondrocytes.

CONCLUSIONS

Se-sensitive PRMT5 increases the half-life of SOX9 protein via PTM and helps to maintain ECM homeostasis of the articular cartilage.

Regulation of Proliferation, Differentiation and Functions of Osteoblasts by Runx2

Runx2 is essential for osteoblast differentiation and chondrocyte maturation. During osteoblast differentiation, Runx2 is weakly expressed in uncommitted mesenchymal cells, and its expression is upregulated in preosteoblasts, reaches the maximal level in immature osteoblasts, and is down-regulated in mature osteoblasts. Runx2 enhances the proliferation of osteoblast progenitors by directly regulating Fgfr2 and Fgfr3. Runx2 enhances the proliferation of suture mesenchymal cells and induces their commitment into osteoblast lineage cells through the direct regulation of hedgehog (Ihh, Gli1, and Ptch1), Fgf (Fgfr2 and Fgfr3), Wnt (Tcf7, Wnt10b, and Wnt1), and Pthlh (Pthr1) signaling pathway genes, and Dlx5. Runx2 heterozygous mutation causes open fontanelle and sutures because more than half of the Runx2 gene dosage is required for the induction of these genes in suture mesenchymal cells. Runx2 regulates the proliferation of osteoblast progenitors and their differentiation into osteoblasts via reciprocal regulation with hedgehog, Fgf, Wnt, and Pthlh signaling molecules, and transcription factors, including Dlx5 and Sp7. Runx2 induces the expression of major bone matrix protein genes, including Col1a1, Spp1, Ibsp, Bglap2, and Fn1, in vitro. However, the functions of Runx2 in differentiated osteoblasts in the expression of these genes in vivo require further investigation.

The role of chondroitin sulfate in regulating hypertrophy during MSC chondrogenesis in a cartilage mimetic hydrogel under dynamic loading

Mesenchymal stem cells (MSCs) are promising for cartilage regeneration, but readily undergo terminal differentiation. The aim of this study was two-fold: a) investigate physiochemical cues from a cartilage-mimetic hydrogel under dynamic compressive loading on MSC chondrogenesis and hypertrophy and b) identify whether Smad signaling and p38 MAPK signaling mediate hypertrophy during MSC chondrogenesis. Human MSCs were encapsulated in photoclickable poly(ethylene glycol) hydrogels containing chondroitin sulfate and RGD, cultured under dynamic compressive loading or free swelling for three weeks, and evaluated by qPCR and immunohistochemistry. Loading inhibited hypertrophy in the cartilage-mimetic hydrogel indicated by a reduction in pSmad 1/5/8, Runx2, and collagen X proteins, while maintaining chondrogenesis by pSmad 2/3 and collagen II proteins. Inhibiting pSmad 1/5/8 under free swelling culture significantly reduced collagen X protein, similar to the loading condition. Chondroitin sulfate was necessary for load-inhibited hypertrophy and correlated with enhanced S100A4 expression, which is downstream of the osmotic responsive transcription factor NFAT5. Inhibiting p38 MAPK under loading reduced S100A4 expression, and upregulated Runx2 and collagen X protein. Findings from this study indicate that chondroitin sulfate with dynamic loading create physiochemical cues that support MSC chondrogenesis and attenuate hypertrophy through Smad 1/5/8 inhibition and p38 MAPK upregulation.

Smad4 deficiency impairs chondrocyte hypertrophy via the Runx2 transcription factor in mouse skeletal development

Chondrocyte hypertrophy is the terminal step in chondrocyte differentiation and is crucial for endochondral bone formation. How signaling pathways regulate chondrocyte hypertrophic differentiation remains incompletely understood. In this study, using a Tbx18:Cre (Tbx18^Cre/+) gene-deletion approach, we selectively deleted the gene for the signaling protein SMAD family member 4 (Smad4^f/f ) in the limbs of mice. We found that the Smad4-deficient mice develop a prominent shortened limb, with decreased expression of chondrocyte differentiation markers, including Col2a1 and Acan, in the humerus at mid-to-late gestation. The most striking defects in these mice were the absence of stylopod elements and failure of chondrocyte hypertrophy in the humerus. Moreover, expression levels of the chondrocyte hypertrophy-related markers Col10a1 and Panx3 were significantly decreased. Of note, we also observed that the expression of runt-related transcription factor 2 (Runx2), a critical mediator of chondrocyte hypertrophy, was also down-regulated in Smad4-deficient limbs. To determine how the skeletal defects arose in the mouse mutants, we performed RNA-Seq with ChIP-Seq analyses and found that Smad4 directly binds to regulatory elements in the Runx2 promoter. Our results suggest a new mechanism whereby Smad4 controls chondrocyte hypertrophy by up-regulating Runx2 expression during skeletal development. The regulatory mechanism involving Smad4-mediated Runx2 activation uncovered here provides critical insights into bone development and pathogenesis of chondrodysplasia.

MicroRNA‑92a overexpression promotes the osteogenic differentiation of bone mesenchymal stem cells by impeding Smad6‑mediated runt‑related transcription factor 2 degradation

Bone mesenchymal stem cells (BMSCs) are an important source of stem cells for tissue repair and regeneration; therefore, understanding the mechanisms that regulate stem cell differentiation in a specific lineage is critical. Runt‑related transcription factor 2 (Runx2) is a bone‑specific transcription factor that serves an important role in promoting osteogenic differentiation. However, Runx2 protein levels are regulated by the ubiquitin‑proteasome pathway. Previous research has identified that Smad6 can interact with Runx2 and enhance Smurf1‑induced Runx2 degradation in a ubiquitin‑proteasome‑dependent manner. Bioinformatics analysis demonstrated that miR‑92a can target Smad6. To characterize the regulatory effect of miR‑92a on osteogenic differentiation of BMSCs and assess the interactive association between Smad6 and miR‑92a, BMSCs were obtained from mice and miR‑92a or Smad6 overexpression vectors were constructed. Reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR) and western blots were used to analyze the expression of miR‑92a and Smad6, and the luciferase reporter assay was used to examine the interaction between miR‑92a and Smad6. BMSCs were induced in osteogenic differentiation media for 21 days. The alkaline phosphatase activity was assessed and Alizarin Red histochemical staining was also performed. The results suggested that the expression of miR‑92a suppressed Smad6‑mediated Runx2 degradation by direct integration with the 3'‑UTR of Smad6 mRNA, which was confirmed by a luciferase reporter assay. In addition, the expression of miR‑92a promoted the osteogenic differentiation of BMSCs. However, the regulatory effect of miR‑92a was inhibited by overexpression of Smad6. Taken together, the results suggest that miR‑92a expression inhibits the osteogenic differentiation of BMSCs by targeting Smad6.

Runx2 mediated Induction of Novel Targets ST2 and Runx3 Leads to Cooperative Regulation of Hypertrophic Differentiation in ATDC5 Chondrocytes

Knowledge concerning expression and function of Suppression of Tumorigenicity 2 (ST2) in chondrocytes is at present, limited. Analysis of murine growth plates and ATDC5 chondrocytes indicated peak expression of the ST2 transmembrane receptor (ST2L) and soluble (sST2) isoforms during the hypertrophic differentiation concomitant with the expression of the hypertrophic markers Collagen X (Col X), Runx2 and MMP-13. Gain- and loss-of-function experiments in ATDC5 and primary human growth plate chondrocytes (PHCs), confirmed regulation of ST2 by the key transcription factor Runx2, indicating ST2 to be a novel Runx2 target. ST2 knock-out mice (ST2−/−) exhibited noticeable hypertrophic zone (HZ) reduction in murine growth plates, accompanied by lower expression of Col X and Osteocalcin (OSC) compared to wild-type (WT) mice. Likewise, ST2 knockdown resulted in decreased Col X expression and downregulation of OSC and Vascular Endothelial Growth Factor (VEGF) in ATDC5 cells. The ST2 suppression was also associated with upregulation of the proliferative stage markers Sox9 and Collagen II (Col II), indicating ST2 to be a new regulator of ATDC5 chondrocyte differentiation. Runx3 was, furthermore, identified as a novel Runx2 target in chondrocytes. This study suggests that Runx2 mediates ST2 and Runx3 induction to cooperatively regulate hypertrophic differentiation of ATDC5 chondrocytes.

Kruppel-like factor 4 regulates matrix metalloproteinase and aggrecanase gene expression in chondrocytes

Kruppel-like factor 4 (KLF4) is a zinc finger transcription factor that plays crucial roles during the development and maintenance of multiple organs. We and others have previously shown that KLF4 is involved in bone modeling and remodeling but roles played by KLF4 during skeletogenesis are still not fully understood. Here, we show that KLF4 is expressed in the epiphyseal growth plate and articular chondrocytes. Most articular chondrocytes expressed KLF4 in embryos but it localized only in a subset of superficial zone cells in postnatal mice. When KLF4 was overexpressed in chondrocytes in vitro, it severely repressed chondrocytic gene expressions. Global gene expression profiling of KLF4-transduced chondrocytes revealed matrix degrading proteinases of the matrix metalloproteinase and disintegrin and metalloproteinase with thrombospondin-1 domain families within the group of upregulated genes. Proteinase induction by KLF4 was alleviated by Trichostatin A treatment suggesting the possible involvement of epigenetic mechanisms on proteinase induction by KLF4. These results indicate the possible involvement of KLF4 in physiological and pathological aspects during cartilage development and maintenance.

DNA methylation of the RUNX2 P1 promoter mediates MMP13 transcription in chondrocytes

The Runt-related transcription factor 2 (RUNX2) is critical for bone formation as well as chondrocyte maturation. Matrix metalloproteinase (MMP)-13 is a major contributor to cartilage degradation in osteoarthritis (OA). We and others have shown that the abnormal MMP13 gene expression in OA chondrocytes is controlled by changes in the DNA methylation status of specific CpG sites of the proximal promoter, as well as by the actions of different transactivators, including RUNX2. The present study aimed to determine the influence of the methylation status of specific CpG sites in the RUNX2 promoter on RUNX2-driven MMP13 gene expression in OA chondrocytes. We observed a significant correlation between MMP13 mRNA levels and RUNX2 gene expression in human OA chondrocytes. RUNX2 overexpression enhanced MMP13 promoter activity, independent of the MMP13 promoter methylation status. A significant negative correlation was observed between RUNX2 mRNA levels in OA chondrocytes and the percentage methylation of the CpG sites in the RUNX2 P1 promoter. Accordingly, the activity of the wild type RUNX2 promoter was decreased upon methylation treatment in vitro. We conclude that RUNX2 gene transcription is regulated by the methylation status of specific CpG sites in the promoter and may determine RUNX2 availability in OA cartilage for transactivation of genes such as MMP13.

Impact of mechanical stimulation on the chondrogenic processes in human bone marrow aspirates modified to overexpress sox9 via rAAV vectors

Background

Evaluation of gene-based approaches to target human bone marrow aspirates in conditions of mechanical stimulation that aim at reproducing the natural joint environment may allow to develop improved treatments for articular cartilage injuries. In the present study, we investigated the potential of rAAV-mediated sox9 gene transfer to enhance the chondrogenic differentiation processes in human bone marrow aspirates under established hydrodynamic conditions compared with the more commonly employed static culture conditions.

Methods

Fresh human bone marrow aspirates were transduced with rAAV-FLAG-hsox9 (40 μl) and maintained for up to 28 days in chondrogenic medium under mechanically-induced conditions in dynamic flow rotating bioreactors that permit tissue growth and matrix deposition relative to static culture conditions. The samples were then processed to examine the potential effects of sox9 overexpression on the cellular activities (matrix synthesis, proliferation) and on the chondrogenic differentiation potency compared with control treatments (absence of rAAV vector; reporter rAAV-lacZ, rAAV-RFP, and rAAV-luc gene transfer).

Results

Prolonged, significant sox9 overexpression via rAAV was achieved in the aspirates for at least 28 days when applying the rAAV-FLAG-hsox9 construct, leading to higher, prolonged levels of matrix biosynthesis and to enhanced chondrogenic activities relative to control treatments especially when maintaining the samples under mechanical stimulation. Administration of sox9 however did not impact the indices of proliferation in the aspirates. Remarkably, sox9 gene transfer also durably delayed hypertrophic and osteogenic differentiation in the samples regardless of the conditions of culture applied versus control treatments.

Conclusions

The current observations show the value of genetically modifying human bone marrow aspirates upon mechanical stimulation by rAAV sox9 as a promising strategy for future treatments to improve cartilage repair by implantation in lesions where the tissue is submitted to natural mechanical forces.

Engineered cartilage regeneration from adipose tissue derived-mesenchymal stem cells: A morphomolecular study on osteoblast, chondrocyte and apoptosis evaluation

The poor self-repair capacity of cartilage tissue in degenerative conditions, such as osteoarthritis (OA), has prompted the development of a variety of therapeutic approaches, such as cellular therapies and tissue engineering based on the use of mesenchymal stem cells (MSCs). The aim of this study is to demonstrate, for the first time, that the chondrocytes differentiated from rat adipose tissue derived-MSCs (AMSCs), are able to constitute a morphologically and biochemically healthy hyaline cartilage after 6 weeks of culture on a Collagen Cell Carrier (CCC) scaffold. In this study we evaluated the expression of some osteoblasts (Runt-related transcription factor 2 (RUNX2) and osteocalcin), chondrocytes (collagen I, II and lubricin) and apoptosis (caspase-3) biomarkers in undifferentiated AMSCs, differentiated AMSCs in chondrocytes cultured in monolayer and AMSCs-derived chondrocytes seeded on CCC scaffolds, by different techniques such as immunohistochemistry, ELISA, Western blot and gene expression analyses. Our results showed the increased expression of collagen II and lubricin in AMSCs-derived chondrocytes cultured on CCC scaffolds, whereas the expression of collagen I, RUNX2, osteocalcin and caspase-3 resulted decreased, when compared to the controls. In conclusion, this innovative basic study could be a possible key for future therapeutic strategies for articular cartilage restoration through the use of CCC scaffolds, to reduce the morbidity from acute cartilage injuries and degenerative joint diseases.